In the manufacture of metal halide lamps, dispensing the required quantity of metal halide salt into the lamp envelope is technically difficult because the salts are highly hygroscopic and the inclusion of water vapor or hydrogen or oxygen in any form within the lamp envelope is highly detrimental. Oxygen present within the envelope may oxidize metals such as tungsten which make up the lamp electrodes, and the resulting metal oxide condenses on the envelope wall and reduces light transmission. When hydrogen is also present, a cyclic action may take place wherein the hydrogen reduces metal oxide on the wall back to metal, freeing the oxygen to attack other electrode metal. This action results in rapid erosion of the electrodes and darkening of the walls to the point where useful life is terminated. In order to forestall or alleviate the foregoing possibilities, highly purified materials must be used for the fill, for instance metal halide salts wherein impurities such as hydrogen or oxygen are held down to a few parts per million, for instance less than 20 parts per million (ppm). In addition, since the metal halides used are highly hygroscopic, it is necessary to handle them in such fashion as to prevent the absorption of moisture or impurities from the atmosphere.
In U.S. Pat. No. 3,676,534--Anderson, "Process Relating to Ultrapure Metal Halide Particles", 1972, there is described a vacuum shot tower technique for preparing purified metal halide salts for use in lamp making, as spheroidal pellets of controlled size. Such pellets can be purchased from Anderson Physics Laboratories, Inc. of Champaign, Illinois and have been successfully used in lamp making. In order to protect the pellets against atmospheric contamination, the practice has been to ship them in sealed glass vials containing several thousand. At the lamp factory, the vial must be broken open in a dry box and the requisite number of pellets manually loaded into lamp bodies by an operator. Such manual handling of the pellets is slow and uneconomical and the operator can easily make an error in the number of pellets loaded into a lamp. Also the risk of contamination through mishap in the dry box, or failure of equipment such as accidental tearing of the hand and arm sheaths of the dry box operator, is ever present. If there is an error in the number of pellets loaded or if the pellets should absorb moisture, lumen output and color may vary from lamp to lamp and efficacy is lowered.